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          <th colspan="3" align="center">Multi-threaded 
        <span xmlns="http://www.w3.org/1999/xhtml">and Multi-process</span>
        Applications</th>
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            <h2 class="title" style="clear: both"><a id="multithread-intro"></a>Multi-threaded 
        <span>and Multi-process</span>
        Applications</h2>
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      <p>
            DB is designed to support multi-threaded  <span>and
            multi-process</span> applications, but their usage means
            you must pay careful attention to issues of concurrency.
            Transactions help your application's concurrency by providing various levels of
            isolation for your threads of control. In addition, DB
            provides mechanisms that allow you to detect and respond to
            deadlocks. 
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      <p>
            <span class="emphasis"><em>Isolation</em></span> means that database modifications made by
            one transaction will not normally be seen by readers from another
            transaction until the first commits its changes.  Different threads 
            use different transaction handles, so
            this mechanism is normally used to provide isolation between
            database operations performed by different threads.
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      <p>
            Note that DB supports different isolation levels. For example,
            you can configure your application to see uncommitted reads, which means
            that one transaction can see data that has been modified but not yet
            committed by another transaction. Doing this might mean your
            transaction reads data "dirtied" by another transaction, 
            but which subsequently might change before that
            other transaction commits its changes.
            On the other hand, lowering your isolation
            requirements means that your application can experience
            improved throughput due to reduced lock contention.
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            For more information on concurrency, on managing isolation
            levels, and on deadlock detection, see <a class="xref" href="txnconcurrency.html" title="Chapter 4. Concurrency">Concurrency</a>.
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